1. Field of the Invention
The present invention relates to a centrifuge rotor, and in particular, to a centrifuge rotor having a predetermined localized region thereon where the occurrence of a failure due to fatigue is most probable.
2. Description of the Prior Art
A centrifuge rotor is a relatively massive member used within a centrifuge instrument to expose a liquid sample to a centrifugal force field. The rotor is provided with a plurality of cavities in which containers carrying the liquid sample are received. The rotor has a central, axial mounting recess provided therein, whereby the rotor may be mounted to a shaft extending from a source of motive energy.
The possibility exists that, in use, the rotor may break apart due either to i) fatigue failure of the rotor material, ii) the imposition of excessive centrifugally induced stresses when the rotor is rotated past its predetermined rated speed (overspeed failure), or iii) failure from the accumulated effects of corrosion caused by sample spillage. A failure produces a number of rotor fragments each of which carries a portion of the kinetic energy of the rotor. A containment system is provided in the centrifuge instrument in order to contain the resultant rotor fragments within the confines of the instrument, thus avoiding damage to people and/or property.
The size of the fragments usually depends upon the cause of the rotor failure. In a rotor failure caused by corrosion, for example, the fragments are relatively small, because the region of the rotor affected by corrosion is the sample receiving cavity near the rotor periphery. Rotor failure caused by fatigue or overspeed may be more severe.
The most severe form of rotor failure is a so-called "bi-hub" failure, in which the rotor breaks into two relatively massive fragments. The origin of the failure in a bi-hub failure is usually in the vicinity of the rotor mounting recess. In such a failure, although the containment system is designed to contain the fragments within the instrument, the impact of the fragments may cause movement of the instrument in the laboratory.
Various forms of mechanical arrangements are known which minimize the possibility of rotor failure due to overspeed. One class of overspeed protection arrangement includes a frangible member which fractures when an overspeed condition is imminent to mechanically disconnect the rotor from its source of motive energy. U.S. Pat. No. 3,990,633 (Stahl), U.S. Pat. No. 4,568,325 (Cheng et al.), U.S. Pat. No. 4,753,630 (Romanauskas), U.S. Pat. No. 4,753,631 (Romanauskas), the latter two patents being assigned to the assignee of the present invention) are representative of this class of overspeed protection arrangement. Another overspeed protection arrangement generally of this form includes a frangible member which fractures when an overspeed condition is imminent to electrically disconnect the rotor from its source of motive energy. U.S. Pat. No. 3,101,322 (Stallman) is representative of this form of arrangement.
Another known overspeed protection arrangement also uses a frangible element on the rotor which fractures when rotor speed reaches a predetermined value. The fragment so produced causes the rotor to be braked by increasing windage within the chamber in which the rotor is carried or by mechanical friction with the surrounding structure, thereby slowing rotor speed. Representative of this class of overspeed protection arrangement are U.S. Pat. No. 4,693,702 (Carson et al., assigned to the assignee of the present invention), U.S. Pat. No. 4,132,130 (Schneider), U.S. Pat. No. 4,509,896 (Linsker), and U.S. Pat. No. 4,507,047 (Coons).
Other arrangements are known which minimize the possibility of rotor failure due to fatigue of the material. One form of such a rotor protection arrangement limits the stress produced in the vicinity of the mounting of the rotor to the shaft. U.S. Pat. No. 4,822,330 (Penhasi) is believed exemplary of this class of device. German Patent 3,806,284 (Hirsch) discloses a centrifuge rotor having portions of the undersurface removed to reduce stress in the rotor.
Another alternative to control of the effects of rotor failure is to design a rotating apparatus, as a flywheel, to exhibit predetermined areas of vulnerability of rupture. The area of vulnerability may be defined by regions of weaker material or by stress risers in the material of the flywheel. Thus, in the event of an overspeed, failure will most likely occur in the area of vulnerability, producing a fragment having a predictable mass. U.S. Pat. No. 3,662,619 (Seeliger) and U.S. Pat. No. 4,111,067 (Hodson) are believed exemplary of this class of device.